This proposal involves the use of quantum mechanical calculations and subsequent analysis to examine the electronic structure of dinuclear and polynuclear Fe-O complexes and Mn-O complexes as relevant models for active sites of metalloprotein systems. Fe-O complexes are found in: (a) ribonucleotide reductase, (b) methane monooxygenase, (c) purple acid phosphatases,and (d) hemerythrin. A polynuclear Mn-O complex occurs at the active site in the oxygen evolving complex of photosystem II. All of our calculations will be of the Xalpha or local spin density self consistent field type. We will systematically study relevant model complexes as a function of oxidation state, coordination geometry, and binding of substrate ligands or reaction intermediates. The calculations will be compared with experimental results by magnetic susceptibility, and by Mossbauer, electron spin resonance and/or optical spectroscopies for proteins and synthetic analogues. Further, we will examine electron density distributions and spin density distributions in some detail since these features should be closely associated with catalytic mechanism. Qualitative and quantitative ligand binding energetics will also be explored. Our goal is to relate predicted and experimental properties as well as to gain insight into catalytic function and regulation in these enzymes.